GRÁFICA 9. RESULTADOS DE LABORATORIO; ESTRUCTURA
12.1 ANEXO 1: LECTURA PRUEBA INICIAL
12.1.3 ANEXO 4 TRANSCRIPCIONES SECUENCIA DIDÁCTICA
After operating for 5 years, the log removal values of FRNA, SC, EC were analysed and reported (Pettigrew et al. 2010). LRVBio and LRVMBR were extracted and alterations were made to permeate concentrations to ensure consistency with the LOD used in this study, effectively censoring some of the older data. Results were compared with LRV PDFs calculated from this study (Table 28).
Table 28 – Comparison of available data at 5 years.
Indicator 5 yearsa 10 years After replacement
min median max 5th median 95th 5th median 95th
FRNA LRVMBR >3.8 >4.5 >4.9 3.7 5.1 6.4 3.4 4.5 5.6 FRNA LRVBio -0.5 0.0 0.6 0.1 0.7 1.3 -0.7 0.7 2.1 SC LRVMBR 3.7 4.6 >4.7 3.6 4.4 5.3 2.8 3.5 4.2 SC LRVBio -1.0 -0.7 -0.5 -0.4 0.1 0.6 -0.9 -0.5 -0.1 EC LRVMBR 5.4 5.7 6.7 5.1 5.8 6.5 6.4 7.1 7.8 EC LRVa Bio 0.0 0.6 0.7 0.5 1.1 1.7 0.1 0.6 1.0
The removal due to biological predation (LRVBio) cannot be mechanistically linked to the replacement of membranes. The change in membranes did result in significantly increased water productivity and as a result HRT was reduced from 17 – 28 to 4 – 11 hr. Although no corresponding operational data is available from the 5 year study of North Head, the design HRT is 5.5 hr. Assuming that hydraulic performances had not declined already at 5 years it is likely that the HRT at 5 years was similar to the value when the membranes were changed over. The results from this study, would suggest some relationship with improved LRVBio at longer HRTs.
Pressure decay testing and visual inspection indicated a significant damage rate on the membrane tested ex-situ. The rate of membrane damage was in the same order of magnitude with previous estimates of 1 fibre breakage per year (Gijsbertsen-Abrahamse et al. 2006). EC (0.5 – 1 μm) and CP (1μm) are typically larger than the membrane pore size (0.04 μm). The significantly enhanced
LRVMem (+1.9 log units) of EC can be explained by the restoration of size exclusion ability of the newly replaced membranes. Clean water testing LRVs for the aged membrane EC and CP at 10 L/m2/hr were 80 and 50% of the corresponding LRVMem medians. SC and FRNA clean water LRVs were 20 and 10% of in-situ LRVMem, respectively. Similar values (0.4 ± 0.1) were reported for MS2
bacteriophage by a microfiltration membrane in clean water (Shang et al. 2005). The presence of suspended solids in an MBR would appear to improve removal by 20 – 50% for larger
microorganisms and up to 90% for smaller bacteriophages, even in the presence of significant membrane damage. The significant increase in membrane permeability upon replacement correlated with a decrease in LRVMem of smaller virus indicators SC and FRNA. Previous studies indicated a reduction in permeability from by 50% corresponded to somatic coliphage removal increases of 1 log (Farahbakhsh et al. 2004), consistent with results presented in this study.
9.5
Conclusions
Over 10 years of MBR operation, membranes become damaged and heavily fouled. The amount of damage would appear to result in loss of size exclusion ability of larger microorganisms; however, in the presence of suspended solids, total loss of size exclusion did not exceed 1.5 LRV or
approximately 20% when compared with new membrane rejection. The presence of fouling significantly improves virus rejection, greater than that possible with virgin membranes, even when membrane integrity is severely compromised. Bio predation limits accumulation of microorganisms rejected by the membrane. The effectiveness of bio-predation is microorganism specific, but in all cases in this study, biopredation was reduced at shorter HRTs. In situations where high log removal value is sought, careful control of operational conditions and maintenance and monitoring of membrane integrity is recommended.
10
Correlation of Turbidity with MBR LRV
10.1
Introduction
Experimental work is underway with the aim of investigating methods to correlate turbidity with LRV. Preliminary aims and findings are included, subject to change, in the following section.
The chosen online monitoring technique must be correlated to LRV. The limitation of the chosen online monitoring technique resolution should yield a maximum demonstrated LRV. The VDoH guidelines have proposed a method for correlation of turbidity with LRV. Previous validation reports have set a critical control limits (CCL) for turbidity at 0.2 – 0.5 NTU, based on research from
membrane suppliers. Generally, when CCLs are exceeded, a timeframe specified to reduce chance of false positive is considered before corrective actions are implemented. Control strategies have included bypass to head of works or waste, or plant shutdown. The critical control limit and corrective actions must be documented as part of the recycled water quality management plan.
An example approach to correlate turbidity, measured in MBR permeate, with LRV is presented below. A minimum of 6 paired samples over different permeate turbidities should be taken of influent and permeate with LRVs calculated with Equation 7. Before attempting to correlate turbidity and LRV, ensure that turbidity meters are cleaned, calibrated and installed as per manufacturer instructions. In order to generate higher turbidities, two approaches are plausible:
• Approach 1: Use a dosing pump to bypass mixed liquor into the permeate line at increasing dosages while noting the bypass ratio. Begin at the lowest ratio and finish at the highest ratio. Correlate the bypass flow with an expected membrane damage rate, based on a flow dilution model. Express LRV and turbidity results as illustrated in Figure 18.
• Approach 2: Sequentially damage membranes by systematically cutting fibres or slicing sheets in order to allow bypass of MLSS particles into the permeate. Record turbidity and damage rate. It may be necessary to backflush membranes with air or liquid to avoid turbidity recovery due to plugging of membrane defects with activated sludge flocs.
After following either Approach 1 or 2, data should be available to allow creation of a plot similar to Figure 18. A CCL for turbidity can then be chosen and a corresponding LRVCCL selected at the point where the LRVC-test correlation meets the chosen turbidity. It is likely that significant loss of resolution will occur at low turbidities. Sampling should only take place where there is a certain measurable change. Before designing a sampling program, it may be worth assessing historical turbidity data to ascertain the normal baseline. The CCL must be chosen within the range of correlated values and greater than the value where loss of resolution occurs. The sampling program should be conducted under the same conservative conditions identified in the validation methodology. Turbidity correlation should be performed at the lowest MLSS concentration in the operating envelope.